The present invention relates to a method for controlling the number of cells in polyurethane foams, in particular when using low pressure mixing heads in the continuous production of flexible polyurethane slabstock foam.
In the production of flexible polyurethane slabstock foam, the foam is normally produced by means of a blowing gas, usually carbon dioxide, which is formed by the reaction between the isocyanate and water. In order to ensure that a homogeneous cell structure is formed, it is necessary to generate gas bubble nuclei in the reaction mixture consisting of the isocyanate component and the polyol component. The quality of the resulting foam, i.e. the uniformity of the foam structure, is substantially determined by the uniformity of the bubble nuclei formed prior to the foaming process. In particular, the number of cells in the foam and their size is determined by the number of the available bubble nuclei. If only a small number of bubble nuclei are available, a foam is obtained with a correspondingly small number of large cells. If a large number of bubble nuclei are available, a foam is obtained with a large number of small cells per unit of volume.
Where high pressure mixing heads are used it has already been proposed to control the number of cells by controlling the content of dissolved gas (in particular air) in one of the components, namely the isocyanate component. The dissolved air is released in the form of fine bubble nuclei as the isocyanate component is injected into the mixing head (see, e.g., published European Application 565,974). The control of a specific content of dissolved air in the isocyanate is however complicated. The '974 published application proposes contacting the isocyanate with the air in a centrifuge, in which the isocyanate acquires a large liquid surface area as a result of the air pressure prevailing in the centrifuge.
According to other proposed methods (German Auslegeschrift 3,920,994, U.S. Pat. No. 4,933,115 and published European Application 239,720) the gas-loading process is carried out in special gas loading devices in which a partial stream of the respective component from a supply vessel is loaded with gas and the partial stream is returned to the supply vessel. The component is fed via metering pumps from the supply vessel to the mixing head. One disadvantage of this method is that the component is loaded with gas on the inlet side of the metering pump. This not only causes considerable metering problems but also damage to the dispersion due to the agglomeration of bubble nuclei as the component passes through the pump. In addition, a highly complicated procedure is required according to the above-mentioned methods for controlling the content of gas in the emulsion.